This application relates to apparatus for synchronizing the aiming and firing of the rapid-fire gun of a microballistic printer or the like.
In copending application Ser. No. 39,372, filed May 15, 1979, I describe a printer which directs a plurality of solid projectiles such as balls about one millimeter in diameter in extremely rapid succession against a printing medium such as a ribbon overlying a sheet of paper. In the gun of the printer, which is movable about orthogonal axes for targeting, balls are introduced successively into a resilient breech which is slightly smaller in diameter than the balls and behind which air is maintained under pressure. The ball is fired by pushing it sufficiently far into the breech to snap the ball through to the barrel side and allow the pressurized air to expand into the barrel and propel the ball outwardly.
In the ballistic printer disclosed in my copending application, it is extremely important that the aiming of the ball gun and the firing of projectiles therefrom be accurately synchronized relative to each other. If the ball gun is moved while a ball is traversing the barrel, the trajectory of the ball is disturbed in an unpredictable manner, causing the ball to strike the medium widely off target. If the aiming and firing steps are poorly synchronized or performed asynchronously, a significant number of balls will miss their mark, giving the sheet a speckled, aesthetically displeasing appearance.
In the previously disclosed ballistic printer, the synchronizing signal is obtained from a disc which rotates on a common shaft with a rotary saw blade used to inject the balls into the ball gun. The disc is formed with a plurality of equidistantly spaced apertures around its periphery equal in number to the teeth on the saw blade. A stationary photodetector disposed adjacent the periphery of the disc generates the synchronizing pulse. Accuracy of synchronization in a system of this type depends, of course, on the registry of the disc apertures with the teeth of the saw blade, which may be difficult to achieve reliably. In such a system, moreover, one is detecting only that a projectile should have been fired, rather than the event that a projectile actually has been fired.